SECTION 317.5. Sludge Processing

(a) General requirements.

(1) Disposal requirements, agreement with. Sludge processing and treatment shall be in agreement with the requirements of the ultimate form of disposal.

(2) Control of sludge and supernatant volumes. Provisions shall be made to insure that waste sludge will be discharged to the sludge digester in such a manner so as to minimize the volume of digester supernatant liquor. Provisions shall be made for the return of supernatant from sludge thickeners and digesters to the head of the treatment works or to the aeration system accounting for the impact on the treatment units.

(3) Piping. All piping from clarifiers to thickeners, digesters, or other sludge processing facilities shall be arranged for ease of maintenance and with sufficient hydraulic gradient to insure the flow of sludge. Piping under stationary structures shall be arranged so that stoppages can be readily eliminated by rodding or with sewer cleaning devices. The sludge piping within the digester, including the sludge drain line, shall be a minimum of four inches in diameter. Appropriate facilities for transfer of supernatant liquor shall be provided. Piping shall include means to observe the quality of the supernatant from each of the withdrawal outlets provided. All units shall be capable of being drained independently of one another.

(4) Sludge pumps. Selection of sludge transfer pumps shall be based on both the quantity and character of the anticipated solids load to be handled by them. Where mechanical pumps are used, a sufficient number of pumps shall be provided so that the design pumping capacity is available with the largest sludge pump out of service. Air lift pumps are an acceptable mechanism for sludge transfer. Duplicate design pumping capacity is not required when air lift pumps are used. Pumps used for pumping sludge shall be specifically designed for that purpose. Centrifugal sludge pumps shall have a positive suction head unless they are self-priming or equipped with some other priming device acceptable to the commission.

(5) Sludge stabilization. Sludge stabilization is required for all biological treatment processes with the exception of extended aeration processes (with a solids retention time (SRT) of 20 days or more) in which case the sludge may be drawn directly to a sludge dewatering facility.

(6) Sizing. Sizing requirements must be determined using the five-day biochemical oxygen demand (BOD₅ ) and design flow of the raw sewage influent to the plant.

(b) Aerobic digesters.

(1) Sludge thickening. Aerobic digesters should be provided with sludge thickening capability.

(2) Aeration. Air supplied from air compressors or blowers through diffusers shall be not less than 30 standard cubic feet per meter (scfm) per 1,000 cubic feet of aerobic digester volume. If a separate system of air compressors or blowers will supply air to the digester, then the compressor or blower system shall be designed so that the air requirements can be met with the largest single unit out of service. If mechanical aerators are used, a minimum of 1.5 horsepower per 1,000 cubic feet must be provided.

(3) Mixing. Adequate mixing of the sludge shall be provided to keep the solids in suspension and to bring the deoxygenated liquid continuously to the aeration device. The amount of mixing shall be based upon the sludge characteristics, the tank geometry, and type of aeration mixing devices.

(4) Volume. A digester shall provide a minimum sludge retention time of 15 days. The design volume of the aerobic digesters may be calculated using 20 cubic feet per pound BOD ₅ per day. This volume should be provided in two cells capable of operating as a single or two-step unit.

(5) Sludge withdrawal. Provisions shall be made to include an effective means of removing solids from the digester.

(c) Anaerobic digesters.

(1) Volume. The following minimum design criteria shall be used in computing the capacity of digesters with and without facilities for heating the sludge undergoing digestion and without sludge thickening ahead of the digester. Variances to the table referenced as follows for minimum digester volume may be granted provided that it can be demonstrated to the satisfaction of the commission that a minimum SRT of 30 days will be provided for unheated digesters and a minimum SRT of 15 days will be provided for heated digesters. Heating of the digester means that adequate facilities shall be provided for heating and mixing the sludge and maintaining a year-round temperature of at least 95 degrees Fahrenheit. Heating coils inside the digester are not acceptable. All heated digesters shall include a thermometer with not less than a four-inch dial to indicate the temperature of digester contents. The use of flat-bottomed digestion chambers is not acceptable. In sewage treatment plants employing sludge thickeners, the volume of the digester may be reduced, with sufficient justification, as a result of the thickeners reducing the volume of sludge going to the digester. The calculations for the required sludge digestion volume shall be based on the minimum percent solids in the sludge expected to be encountered.

Attached Graphic

(2) Mixing. Adequate mixing of digester contents is required for all first-stage and all single-stage digesters. Mixing may be performed by mechanical equipment, including external pumps, or by gas recirculation. The rate of mixing shall be such that the flow created in the digester is sufficient to completely mix the incoming sludge with the digester contents and prevent the formation of a scum layer.

(3) Digester covers. Uncovered anaerobic digesters are not acceptable. The sludge and supernatant withdrawal piping for all single-stage and first-stage digesters with fixed covers shall be arranged in such a manner so as to minimize the possibility of air being drawn into the gas chamber above the liquid in the digester. All digester covers shall include a gas chamber adequate for the gas production anticipated. Digester covers shall be gas tight and the specifications shall require a test of every digester cover for gas leakage.

(4) Gas piping and safety equipment. The gas piping shall be adequate for the volume of gas to be handled and shall be pressure tested for leakage (at 1.5 times the design pressure) before the digester is placed into operation. All gas piping shall slope at least 1/8 inch per foot to provide drainage of condensation in the gas piping. The main gas line from the digester shall have a sediment trap equipped with a drip trap. Drip traps shall be provided at all other low points in gas piping. The gas piping to every gas outlet including the pilot line to the waste gas burner shall be equipped with flame checks or flame traps. A natural or bottled gas source shall be utilized for the burner pilot. Flame traps with fusible shutoffs shall be included in all main gas lines. The gas line to the waste gas burner shall include a suitable pressure, vacuum and relief valve. Digester covers shall be equipped with an air vent which includes a flame trap, a vacuum breaker, and pressure relief valve. The main gas line shall be provided with a manometer or other acceptable device which measures the gas pressure in inches of water. Manometers may be used to measure the gas pressure in other gas lines. All manometers shall be vented to the atmosphere outside digester buildings. A gas meter to measure the rate of gas production is desirable and is mandatory on all anaerobic digester systems designed for 1.0 mgd facilities or larger. All rooms in digester buildings with floor level below grade shall be ventilated. Ventilation may be either continuous or intermittent. Ventilation, if continuous, shall provide at least six complete air changes per hour; if intermittent, at least 30 complete air changes per hour.

(5) Other requirements. The discharge end of sludge inlet piping shall be separated from the overflow of the supernatant liquor withdrawal point by a minimum distance equal to the radius of the digester tank. Every digester shall be provided with an overflow. A means shall be provided by which the level can be varied from which supernatant liquor is withdrawn either automatically or by the operator. If this means is by withdrawal pipes at different levels in the digester, at least three different levels of supernatant liquor withdrawal shall be provided. All supernatant liquor withdrawal systems shall be provided with sampling cocks or other means of inspecting and testing the supernatant liquor from each level. Piping for hot water heating systems may be of any size adequate for the flow. The fresh water supply to hot water heating systems shall be from a tank with an air gap between the top of the tank and the fresh water supply pipe to prevent a cross connection between the digester hot water system and the fresh water supply system.

(6) Treatment of digester supernatant liquor. Supernatant liquor from anaerobic digesters may be treated by chemical means or other acceptable methods before being returned to the plant. If the commonly used method of dosing with lime is employed, the following criteria shall apply: lime shall be applied to obtain a pH of 11.5. The lime feeder shall be capable of feeding 2,000 mg/liter of hydrated lime or its equivalent. The lime shall be mixed with the supernatant liquor by a rapid mixer or by agitation with air in a mixing chamber. After adequate mixing, the solids shall be allowed to settle. The supernatant liquor treatment system may be a batch or continuous process. If a batch process is used, the mixing and settling may be in the same tank. The sedimentation tank shall have a capacity to hold 36 hours of supernatant liquor but not less than 1.5 gallons per capita. If a continuous process is used, the sedimentation tank shall have a detention time of not less than eight hours. Solids settled from the supernatant liquor treatment are to be returned to the digester or conveyed to sludge handling facilities. The clarified supernatant liquor shall be returned to the head of the treatment works or to the aeration system.

(d) Other stabilization processes.

(1) Incineration and heat treatment. The equipment shall be housed in a fireproof building. Adequate facilities shall be provided for storage of sludge during the longest period that drying and or incineration units might normally be out of service for repairs or maintenance. Plans for control of odors, insects, fly ash, and for adequate facilities for the disposal of dried sludge or ash shall be provided to the commission. Prior to construction of an incineration or heat treatment facility, consultation should be made to the Texas Air Control Board for applicable emission standards and the possible requirement for a separate Texas Air Control Board permit.

(2) Composting, wet oxidation, and other processes. Design information given to the commission shall include the demonstrated level of stabilization achieved by the process to be employed. Test results to verify the degree of stabilization may be required. In addition, design information shall address design and/or operational methods to minimize odor, insects, and other nuisance conditions. Sludge storage requirements for each process shall be provided to the commission. Also, the ultimate disposal method for the processed sludge shall be reflected in the waste disposal application.

(e) Sludge dewatering facilities. Sludge shall be dewatered sufficiently to meet the requirements of the ultimate form of disposal.

(1) Sludge drying beds.

(A) Required area. The area of sludge drying beds to be provided will vary in accordance with the average rainfall, average humidity, and type of treatment process used. The required area for aerobic sludge dewatering shall be determined from §317.12 of this title (relating to Appendix D) (for anaerobic sludge dewatering, the value obtained from §317.12 of this title) may be reduced 35% to determine the required area) using a waste load based on sewage strength and the daily average flow of the raw sewage. The bed area sizing requirements shown in §317.12 of this title are for sludge drying beds utilizing a continuous underdrain media as specified in this subsection. Concrete (or similar impervious material) sludge drying beds which do not use an underdrain media may require additional area and will be evaluated on a case-by-case basis; however, in those counties of the state which experience both high rainfall and high relative humidity (Brazoria, Chambers, Fort Bend, Galveston, Hardin, Harris, Jasper, Jefferson, Liberty, Newton, and Orange), other methods of sludge dewatering should be utilized in lieu of sludge drying beds. Where sludge drying beds are used in those counties of high rainfall and humidity, provisions shall be made in the design of these beds for covering the beds, for means of accelerated dewatering, or for extra storage capacity and alternate dewatering methods to effectively dewater the sludge during inclement weather.

(B) General design features. At least two sludge drying beds shall be provided and they shall be constructed at elevations above groundwater level. Construction shall be such as to exclude surface water runoff from the beds and seepage from the beds into the ground. Channels shall be of sufficient grade and size to facilitate the flow of the sludge to the various beds. Runners should be provided to facilitate sludge handling.

(C) Filtrate. The filtrate (or drainage) from the sludge drying beds shall be returned to the head of the treatment works or to the aeration system.

(D) Sludge removal. A splash block or slab shall be provided at the point where digested sludge is discharged onto each of the beds. Appropriate means shall be provided to facilitate the removal of the dried sludge from the beds for disposal without bed damage resulting. Every sludge drying bed should include a removal gate or stop planks in one end to provide access for machinery and trucks to remove and haul away the dried sludge.

(E) Media. A minimum depth of 12 inches of filtering material, of which four to six inches is coarse sand, is required. To exclude surface water and eroded earth, the bed shall be protected by a permanent wall which shall extend at least 12 inches but not more than 24 inches above the finished surface of the beds.

(2) Vacuum filters, belt filters, belt filter presses, and other mechanical dewatering filters.

(A) Multiple units. Where dewatering of sludge is proposed, the design engineer shall provide data to document sufficient capacity, alternate disposal means, or storage facilities capable of maintaining normal daily operations during breakdowns, upsets, etc.

(B) Filtrate. The filtrate from the filters shall be returned to the head of the treatment works or to the aeration system. Consideration shall be given to the impact of the returned filtrate on the treatment units and to providing odor and insect control facilities.

(3) Portable dewatering units. If sludge is to be treated using portable mechanical dewatering units, provisions shall be made in the facility plan or preliminary engineering report for the location and connection of the portable dewatering unit(s) during facility operation.

Source Note: The provisions of this §317.5 adopted to be effective November 26, 2015, 40 TexReg 8341